In a large frying pan, warm the olive oil over low heat. Add the onions to the oil and cook gently for 30 minutes.
Add the vinegar and sugar and season with salt and pepper. Mix well and let the onions cook for 30 to 45 minutes, stirring often, or until the onions begin to lose their form and become caramelised. Add a little water from time to time if the mixture dries out too much.
Serve on crackers, crostini or bread, topped with a little gorgonzola or goat cheese.